More rework on the groups system.

* BESTOWGROUPROOT permission added to dbdefs.

* Permissions criteria for group operations changed in dbdefs
  (consult code for full explanation.)

* Approveuser and Editgroup now check for BESTOWGROUPROOT
  permissions before allowing changes to group_root.

* approveuser_form and editgroup_form do not show "Group Root"
  as an option unless you are allowed to set it (or it is already set.)

* editgroup does not UPDATE rows where trust has not been changed.

* showgroup does a correct check to see whether to show the
  "group options" subpage.